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The material for the thermocouple protection tube must be selected by taking into account the corrosion of the thermocouple under conditions such as high-temperature oxidation, carbonization, nitridation, sulfidation, and chlorination. The choice is made based on principles of cost-effectiveness, ensuring accurate temperature measurement, and extending the service life of the thermocouple as much as possible. Although this process seems simple, it actually involves a high level of technical expertise. Changhui Instruments has accumulated extensive experience in the research of thermocouple temperature measurement technology, which is shared with you in this article. The operating environment of thermocouples involves various atmospheres and stress conditions. High-temperature components in industries such as energy, petrochemicals, building materials, machinery, metallurgy, and aviation are often exposed to various corrosive environments. The corrosion of thermocouples can be classified according to the environment in which they are located, as shown below: http://yunrun.com.cn/upload/201906/02/201906020010287276.png Corrosion of thermocouples in high-temperature oxidation environments. High-temperature environments are often associated with oxidation; therefore, the material used for the protective casing of thermocouples must have sufficient oxidation resistance. Typically, Cr2O3 formed at high temperatures can meet the requirements of high-temperature environments. Therefore, heat-resistant steels and heat-resistant superalloys contain large amounts of chromium, which enables the material to form a dense Cr2O3 protective layer at high temperatures. Fe-Ni-Cr and Ni-Cr series heat-resistant steels and alloys with a w(Cr) of 16%~30% can be used at temperatures up to 1000°C in high-temperature oxidation and corrosive environments. However, when the temperature is above 1000°C, Cr2O3 peels off severely; at this point, the oxide layer of aluminum provides better protection than that of chromium. In particular, the dense a-A1203 formed on the alloy surface maintains high stability even at 1300°C. Furthermore, it has been found that the stability of fluorides depends to a large extent on the oxygen partial pressure in high-temperature environments and the equilibrium oxygen partial pressure during oxide formation. Therefore, in environments with high temperatures and low oxygen partial pressures, heat-resistant steels and heat-resistant superalloys that have an extremely low equilibrium oxygen partial pressure at high temperatures and can form a dense Al2O3 layer should generally be chosen. Tables 1 and 2 list the partial pressures of gases and the equilibrium oxygen partial pressure of oxides in different industrial environments. The most straightforward way to determine the stability of an oxide is to consult a graph showing the relationship between the standard formation free energy of the oxide and temperature. At the same temperature, the lower the position of the oxide, the greater its stability. Table 1 Oxygen partial pressure in different industrial environments. High-temperature environments: Oxygen partial pressure/×101 kPa – Petrochemical industries: 10-25~10-20; Gas cooling towers: 10-25~10-20; Fluidized beds: 100~10-5; Coal gasification: 10-25. Table 2 Equilibrium oxygen partial pressures of oxides. Oxides: Oxygen partial pressure/×101 kPa – (2/3)Al2O3: 1.6×10-34; (2/3)Al2O3: 3.4×10-22; (1/2)Fe3O4: 5×10-14 (at 950°C); 2NiO: 9×10-11 (at 950°C); (2/3)Fe3O4: 3×10-10 (at 950°C). Corrosion of thermocouples in high-temperature carburization environments: Carburization often occurs in various metal materials used in petrochemical, metallurgical, and industrial heating furnaces. It manifests in two ways: when the activity of carbon in the environment is below 1, carbon in the atmosphere diffuses into the metal, forming internal carbides, which thereby reduces the ductility of the alloy. Carbide corrosion mainly depends on the solubility of carbon in the alloy and its diffusion rate; therefore, it occurs primarily at high temperatures above 1000°C. In this environment, nickel-based superalloys are generally used. The element silicon can reduce the solubility and diffusion rate of carbon, while an increase in the amount of chromium dissolved in the alloy facilitates the formation of dense Cr2O3, which effectively enhances the alloy’s resistance to carburization. However, the oxygen partial pressure in the petrochemical industry is generally low; therefore, aluminum-containing heat-resistant superalloys are often used to form an Al2O3 protective layer that prevents internal carbonization. In iron, cobalt, and nickel-based alloys exposed to a mixed gas of CO + H2(+H2O), metal dusting occurs when the activity of carbon in the environment is greater than 1. Studies have shown that when alloy materials are exposed to an atmosphere containing supersaturated carbon, they will absorb high concentrations of carbon after a certain period of time. If deposited graphite forms on the metal surface, its activity drops to 1, thereby causing the metal matrix containing supersaturated carbon to decompose. In iron-based alloys, the metastable intermediate phase Fe3C is formed first, and then it decomposes into graphite and metal particles. In nickel-based alloys, graphite forms and grows into the solid solution, thereby damaging the metal. The pulverized metal particles will further catalyze carbon deposition. Therefore, once ashing begins, its reaction can produce a large amount of carbon. Compared to iron-based alloys, nickel-based alloys have stronger resistance to graying. Furthermore, as the chromium content increases, the protective layer formed in the superalloy becomes denser, resulting in greater resistance to aging. Corrosion of thermocouples in high-temperature nitridation environments. Nitridation atmospheres are commonly found in high-temperature environments such as nitrogen, nitrogen-hydrogen, and ammonia. Similar to metal carbidization, in such environments nitrogen can penetrate into the metal to form metal nitrides (usually chromium nitrides), which reduces the toughness of the material or causes cracking along the grain boundaries. Nitrogen has a low solubility in nickel-based alloys; therefore, these alloys are not sensitive to internal nitridation. In contrast, Fe-Ni-Cr alloys (such as 800H and DS) undergo internal nitridation at 1000°C, which reduces the toughness of the material. Table 3 shows the changes in impact toughness over 1000 hours at 1200°C in a 10% H2/N2 atmosphere for several common heat-resistant steels and heat-resistant nickel-based alloys, as determined through tests conducted by Changhui Instruments. As can be seen from the table, in a nitridation atmosphere, alloy 600H exhibits the best resistance to nitridation. Table 3 Changes in impact toughness (V-notch) of heat-resistant metal materials. Name of VDM, Type of alloy, German designation, ak/(J/cm2) (before use), ak/(J/cm2) (after use): Cronifer 2520 alloy 314: 1.4841, 275; 10. Nicrofer 3220 H alloy 800H: 1.4958, 385; 13. Nicrofer 7216 H alloy 600H: 2.4816, 345; 238.5. Nicrofer 6023 H alloy 601H: 2.4851, 194; 83. Nicrofer 6025 HT alloy 602CA: 2.4633, 90; 78.5. Corrosion of thermocouples in high-temperature sulfidation environments. Sulfidation atmospheres are commonly found in the chemical industry and in coal conversion processes. When the sulfur content is very high, sulfides form on the metal surface. Its growth rate is several orders of magnitude higher than that of oxides, and it often forms low-melting-phase compounds. For example, nickel-based superalloys often form eutectics in high-sulfur environments at a temperature of only 635°C, which causes the properties of the alloy material to deteriorate rapidly. The use of GH3039 high-temperature heat-resistant alloy as a thermocouple protection tube leads to its rapid melting in high-sulfur atmospheres, which is due to an inappropriate choice of material. In an atmosphere containing SO2, the resistance to sulfidation/oxidation increases as the Cr content rises. If the material is to operate at high temperatures under vulcanization conditions and is frequently cooled to room temperature in humid, sulfur-containing acidic environments, Nicrofer 3033 and alloy33 (1.4591) are generally used. Due to a w(Cr) level of 33%, this material exhibits good resistance to sulfidation and acid corrosion. Reducing sulfur-containing gases, such as those in the coal gasification process, cause significant damage to metal materials. Experimental studies show that in Ni-Fe-Cr alloys, the w(Cr) content must be at least 25% in order to withstand the erosion caused by sulfur during coal gasification ； It was also found that the addition of Si can further improve the sulfidation resistance of the alloy. Corrosion of thermocouples due to high-temperature chlorination: When metal materials are exposed to chlorine (Cl), hydrochloric acid/hydrogen chloride (HCl), or other halogen-containing environments, metal halides and intergranular corrosion caused by Cl and F occur. In an oxygen-poor HCl atmosphere, chlorides of iron, nickel, and chromium may form. Since most metal chlorides have high vapor pressures even at moderate temperatures, the evaporation of these metal chlorides is likely to occur. Iron chlorides are formed at a temperature of just 167°C. Due to the very low vapor pressure of nickel-iron chlorides, nickel-based alloys possess resistance to corrosion by various halogen elements. For example, Ni-Cr-Mo alloys have the best resistance to chlorination. Nicrofer 6020-Alloy 625 (2.4859) and Nicrofer 6616-Alloy C-4 (2.4610) maintain their corrosion resistance even at temperatures of 850°C in a 10% HCl/H2 atmosphere. In a stable low partial pressure atmosphere, pure nickel (alloy 200) and Ni-Mo alloys (Nicrofer 6928-alloy B-2) can be used. Chloride-induced oxidation is often observed during deposition in an oxygen-containing Cl2/O2 atmosphere or with chlorides. This is the main mode of corrosion in the boiler pipes of waste incinerators. In this environment, alloys such as Nicrofer 7216-alloy 600H (2.4816), Nicrofer45-TM, and alloy45-TM 600H (2.4889) exhibit excellent corrosion resistance. It is particularly important to note that alloys such as Nicrofer 6025 HT, which form alumina, can be used in oxidizing atmospheres containing Cl. However, it is not suitable for use in an oxygen-poor HCl atmosphere, as such alloys tend to form aluminum chloride (AlCl3), and they are prone to flaking and evaporating at very low temperatures (173°C). Source: Changhui Instruments http://yunrun.com.cn/